The following relates to the water heater arts, water heater control arts, water heater maintenance arts, and related arts.
Water heaters are ubiquitous appliances in residential and commercial settings, used to provide hot water for washing, cleaning, laundry processing, industrial processes, and so forth. A typical electric water heater includes a water storage tank with one or more heating elements, typically at upper and lower positions. Cold water enters near the bottom of the water storage tank via a cold water feed pipe, and is heated by the heating elements. Heated water loses density, causing it to tend to rise upward, and this flow pattern is reinforced by entry of cold water near the bottom of the tank and extraction of hot water from the top of the tank. A gas water heater operates similarly, with the resistive electrical heating elements being replaced by a gas burner usually located near the bottom of the water storage tank. In either case, temperature control is typically achieved by a simple thermostat-based controller that applies heat when the water temperature in the storage tank falls below a deadband minimum and turns off the heater (gas or electric) when the water temperature rises above a deadband maximum. Within the deadband the heater setting remains unchanged, producing a temperature cycling within the deadband (possibly with some overshoot and/or undershoot) about a temperature set point located at about the middle of the deadband. This type of control advantageously leverages thermal hysteresis to reduce the on/off cycling of the heating element. Water temperature is usually set by adjusting the set point, with the deadband limits defined relative to the set point (e.g., ±2° C. above/below the thermostat set point).
Recognizing that water heaters in a building, city, or region represent a large distributed thermal energy storage reservoir, there has been interest in leveraging aggregations of water heaters as energy storage devices to provide demand response, in which the electrical load of the electric grid is matched with electrical generation. (By comparison, conventionally the power generation is adjusted to match load, for example by bringing ancillary power generators online/offline as needed to match load). By way of illustration, to perform load shedding the water heater operation can be curtailed during peak energy usage periods, with hot water continuing (for a time) to be available from the hot water tank. As another illustration, in frequency control the load is adjusted at a higher frequency, typically on the order of seconds, in accord with an Automatic Generation Control (AGC) signal to maintain the grid frequency.
To perform demand response, especially at higher frequencies such as those required for AGC-based frequency control, the water heaters typically must be controlled remotely, for example by retrofitting the water heater with a remotely operable load controller (or, in the case of a new water heater, including such a load controller as an original manufacturer component). Also, the demand response must be balanced against the traditional function of water heaters: to provide hot water (which limits the time that the water heater can be kept off), as well as safety considerations such as not overloading the electrical circuits, or generating water that is scalding hot (which limits the time the water heater can be kept heating). To balance these considerations, it is useful to provide feedback to the aggregate controller, such as the water temperature in the storage tank, instantaneous water heater power consumption, or so forth.